Coupe rear window question

If it feels like it may break, don't force it or you will break something. The only way I know to fix it is to remove the rear seat to remove the side panel. I haven't done yet on my 65 Dart, so can't detail. On other cars, one removes the seat back first. It hooks at the top and is usually secured by bolts at the bottom (feel for them). The bottom then pulls out easy, and the panel snaps off (careful since it is just hardboard, use a pry tool).

Once you get inside, you can trouble-shoot. Often the plastic button that secures the glass falls out and the glass can move around and wedge in the tracks. If not, the roller channels may just need cleaning and fresh grease. The rollers are plastic (I think) and may have broken. Finally, the glass may bind in the side channels. There are usually plastic inserts that degrade. Silicone spray usually helps, which you can do without any disassembly, but unlikely that alone will fix it.